The adaptation of an oilfield gate valve for long unattended service requires changes in the basic design of the valve. A typical through-conduit, rising stem, gate valve includes a valve body having a flow way through which the pipeline fluids pass and a chamber intersecting the flow way interiorly of the valve body. A gate is disposed within the chamber for reciprocation across the flow way where in the lower position, for example, a flow port in the gate registers with the flow way to permit line fluids to flow through the valve and in the upper position the gate blocks flow through the flow way. The gate is reciprocated within the valve chamber by an actuator which may be electrical, hydraulic, or pneumatic causing the gate to move across the flow way to open and close the valve.
In oilfield operations it is desirable for gate valves in long unattended service to have "fail-safe" valve operators. A fail-safe operator returns the valve to the safe (generally closed) position upon sensing a failure of its power source such as loss of electrical potential or hydraulic or pneumatic pressure. This means that in a fail-safe operator, the power source must be applied to the operator continuously when it is in the "unsafe" position.
Generally conventional subsea fail-safe gate valves rely upon hydraulic actuator pressure to move the gate downwardly in the valve body and upon internal valve pressure to move the gate upwardly after loss of hydraulic pressure. A spring is disposed in the actuator cylinder to assist the force of the internal valve pressure on the stem urging the gate upwardly to insure that the valve will fail-safe close when internal valve pressure is very small. The position of the gate without actuator pressure is the "normal" or "safe" position of the valve and may be open or closed depending upon the location of the flow port in the gate. For further discussion of the design of high-pressure valves for unattended service, reference is made to the Natho U.S. Pat. Nos. 2,885,172 and 2,974,677 and the article "Development of High-Pressure Valves for Unattended Service" by John H. Fowler published by ASME in 1967.
Fail-safe gate valves with pneumatic or hydraulic actuators have been used for years on conventional land and offshore Christmas trees. Subsea drilling from floating vessels caused trees and BOP stacks to become located on the ocean floor. It was a logical extension to use these conventional fail-safe valve designs for BOP stack valves, production valves, or tree production valves in subsea operations.
Remote subsea fail-safe gate valves are most often used with hydraulic actuators. The hydraulic actuators and often their controls are located at the ocean floor. One of the reasons for such location is the effect of the hydrostatic head, i.e. ambient sea pressure, on the actuators. Although the location, configuration and types of actuators and controls vary, their operation is subjected to ambient sea pressure whether the actuator and controls are a closed or open hydraulic system.
The time required for a fail-safe valve to fail-safe close is critical and a short response time is desirable to reduce the amount of throttling as the valve moves into the normal position. For most fail-safe valves, fail-safe closure occurs without difficulty at depths commonly encountered, and successful use of these fail-safe valves in water depths to 1,000 feet has been common for years. However, as water depth increases, increased hydrostatic head, or ambient sea pressure, creates forces on the valve not normally considered in valve designs for common depths. The design of these valves approaches a threshold whereby a combination of conditions can unreasonably delay or preclude the fail-safe closure of this type of gate valve. Such standard fail-safe valves may be delayed in closing or fail to close under supposed fail-safe conditions at these increased depths upon loss of hydraulic control pressure coupled with low internal valve line pressure when the ambient sea pressure becomes greater than the internal line pressure or internal valve pressure of the valve.
Both sides of the valve piston of a conventional fail-safe valve are subjected to ambient sea pressure since the piston is part of the hydraulic actuator and controls for the valve. Where the ambient sea pressure becomes greater than the upstream line pressure at increased depth, a downacting force on the piston is created by the ambient sea pressure tending to keep the valve open even if hydraulic actuator pressure is lost. This downacting force due to the hydrostatic head is the difference between the ambient sea pressure and internal valve pressure multiplied by the piston's stem diameter. When this downacting force becomes greater than the spring load on the piston, the valve will remain open and fail to close under fail-safe conditions.
As valve size increases it becomes even more impractical to increase the spring load to overcome the downacting force caused by increased ambient sea pressure. The use of a precharged accumulator in place of the spring also may not be satisfactory since long-term use in unattended service requires better reliability than typically has been demonstrated by precharged accumulators in actual subsea service.
This unwanted force resisting closure in conventional fail-safe valves caused by increased ambient sea pressure has been eliminated by attaching a lower stem to the gate and permitting the ambient sea pressure to act on both the upper and lower stem areas. Thus the ambient sea pressure and internal valve pressure act on the upper stem with an equivalent opposing force on the lower stem providing a zero net force on the gate which now depends solely on the spring force for closure. However, in this design the gate drag becomes exceedingly large and may be impractical where the differential pressure across the gate approaches the upstream pressure.
One solution to this problem is described and claimed in U.S. Pat. No. 3,933,338 issued Jan. 20, 1976 to Herd, McCaskill and Childers entitled "Balanced Stem Fail-Safe Valve System". That patent discloses the use of a lower stem which can contact, but is not connected to, the gate. The upper end of the lower detached stem is subjected to internal valve pressure and the bottom end is acted on by the ambient sea pressure which also acts on the upper stem attached to the gate. Where the internal valve pressure is greater than the ambient sea pressure, the lower detached stem remains pushed down and does not engage the gate permitting the valve system to function as a conventional fail-safe valve. Hence it plays no part in valve functions. However, in the fail-safe mode, i.e. the loss of control pressure combined with low internal valve pressure, the lower detached stem moves upwardly to contact and exert force on the gate balancing the unwanted force from the ambient sea pressure on the upper stem. For further design information on such valves, reference is made to the article "How to Make a Valve Which Will Fail-Safe in Very Deep Water" by D. P. Herd and J. W. McCaskill published in 1976 by the ASME.
However, such fail-safe valves designed for increased ambient sea pressure are still subject to pressure lock. Pressure lock occurs when line fluids in the valve chamber, for some reason, cannot be displaced during the reciprocation of the gate. Resistance to the displacement of the line fluids delays opening and if great enough, may prevent the valve from opening at all.
Under conventional conditions, pressure lock does not pose a problem to either unbalanced or balanced fail-safe valves. In a fail-safe valve having no lower gate stem the line fluids in the valve chamber are displaced between the upstream side of the gate and valve body to avoid pressure lock and in a fail-safe valve having a lower gate stem extending into a bore in the lower valve body and in sealing engagement therewith, the line fluids are displaced through a port extending from the bore below the lower stem to the flow way. See publications on the McEvoy Model "EU" Straightway Valve including the 1976-1977 Composite Catalog of Oilfield Equipment and Services. Thus in either case as the gate or lower stem reciprocates within the valve body, means are provided to permit displacement of line fluids within the valve chamber or bore.
In many subsea drilling and production operations two fail-safe valves, such as disclosed in the above Herd et al patent, are disposed in series on the subsea BOP choke or kill line and may be subjected to potential pressure lock conditions. The valve adjacent to the BOP stack is called the inboard valve and the other valve, generally connected to the "choke or kill" line extending to the surface, is the outboard valve. The valves are generally connected hub to hub or with only a short section of flowline disposed between the two valves creating a small flow bore volume. Where the flowline fluid is incompressible, the relatively small finite flow bore volume, when compared to the volume of the production line or choke and kill line, may preclude the inboard valve from displacing internal valve fluids downstream between the inboard and outboard valves where the gates seal around the flow ways of the valves and the flow bore is filled with the incompressible flowline fluid thus causing the inboard valve to pressure lock and fail to open.
For example, in testing the pipe rams of a blow-out preventor, a test plug is run into the subsea well to seal off the well at the wellhead. The pipe rams are activated and the inboard and outboard valves on the choke line are closed. The kill line is pressurized to test the pipe rams. If the short flow line between the inboard and outboard valves also becomes pressurized due to leakage of the inboard valve or because the inboard valve is closed during pressurization for example, and the pressure in the kill line is bled off rapidly, pressurized incompressible fluid, such as fresh or salt water, will become trapped in the short flow line. As the pressure in the kill line drops, the pressure in the short flow line causes the gate of the inboard valve to seal on the blow-out preventer side of the inboard valve and the gate of the outboard valve to seal on the choke line side of the outboard valve. This situation prevents the displacement of internal valve fluids of the inboard valve upon actuation and causes it to pressure lock and fail to open. The internal valve fluids cannot be displaced on the one side because of the sealed gate and they cannot be displaced on the other side because of the incompressible fluid in the short flow line between the valves.
Fluid pressure may also become trapped between the valves during the testing of the inboard and outboard valves themselves. It is not uncommon to close one of the valves and apply fluid pressure through the choke or kill line to test the seals. If the other valve is closed and the choke or kill line is bled rapidly, fluid pressure may become trapped between the valves due to the gates sealing the fluid pressure in the short section.
The same phenomenon may occur in gate valves having a split gate. Where fluid pressure is trapped between the gate halves causing a seal on both the upstream and downstream sides of the valve by the gate segments, the valve can pressure lock itself where the internal valve fluid cannot be displaced thereby preventing the valve from opening.
The invention overcomes the pressure lock and fail-safe closure defects seen in prior art valves operating in series or with a split gate on a submerged flowline controlling the flow of an incompressible fluid over 1,200 feet below sea level.